1. Field of the Invention
The present invention relates generally to wireless stations and networks, and more particularly to a method of scanning channels in a wireless network.
2. Description of the Related Art
Regulatory domains, such as individual nations, independently determine the frequency band and the maximum transmission power allowed for wireless communication systems. The conditions established by each regulatory domain may vary significantly even for the same wireless communication system. For example, while the 4.9-5.0 gigahertz (GHz) band is allowed for IEEE802.11a wireless local area network (WLAN) in Japan, the 4.94-4.99 GHz band is reserved for public safety band in the United States, and thus cannot be used for IEEE802.11a. Similarly, the 5.470-5.725 GHz band, which is planned to be used for IEEE802.11a WLAN in Europe, overlaps with a military band in the United States.
In contrast to an access point which is stationary and can be well controlled by the regulation of the corresponding domain, wireless stations may move through different domains around the world, and thus will be under different regulations depending on the domain. If an IEEE802.11a station of Japan or Europe is moved to the United States, for example, it should be able to get the correct information on the frequency band and the maximum transmission power permitted in the United States, enabling the wireless station to adapt itself to be in compliance with the regulations of the United States.
Generally, wireless stations use channel scanning to get such information as the frequency band and the maximum transmission power. Channel scanning can be classified into two categories: passive channel scanning and active channel scanning. A wireless station can passively scan channels by just receiving a beacon frame of each channel. As shown for example in FIG. 1, beacons are transmitted with an approximate period of the beacon interval. Assume that a beacon frame from an access point is received for a specific channel. Since an access point, which is more properly regulated by each domain, is using the channel, the wireless station can determine that the scanned channel is within the legal frequency band of the residing domain. Furthermore, the beacon frame may have an information field that indicates the maximum transmission power for the channel, which will be used by the wireless stations in their power control.
However, if a beacon is not received for the scanned channel within a certain time limit after a wireless station starts to listen for a beacon, then the channel is determined by the wireless station to be unavailable for one of two reasons. One reason is that the channel is outside of the legal frequency band of the residing domain. The other is that there are currently no access points using the channel even though the channel is within the legal frequency band.
By listening to all channels that are used in the WLAN, a wireless station can collect all the information on channels that are currently supported by access points and within the legal frequency band of the residing domain. Since passive channel scanning is performed using only the receiving capabilities of wireless stations, there is no risk of violating the regulation of any domain. In this aspect, passive channel scanning is suitable for safe domain-aware channel scanning. However, passive channel scanning is disadvantageous because it may take a long time, which can be as much as a beacon interval of the corresponding access point. For IEEE802.11a, the beacon interval is usually set on the order of 100 msec. As a result, passive channel scanning may not be appropriate for channel scanning as, for example, in a handover where fast channel scanning is particularly important.
In contrast, active channel scanning involves the generation of probe frames and the subsequent processing of received probe response frames for each channel. FIG. 2 shows a timing diagram of a probe frame and a probe response frame. If the network is not crowded, the probe frame and the probe response frame can be transmitted with awaiting time as small as the distributed interframe space (DIFS), which is a very small value compared to the beacon interval. Assume that a probe response frame from an access point is received for the channel on which a probe frame was sent. Since an access point, which is more properly regulated by each domain, is using the channel, the wireless station can determine that the scanned channel is within the legal frequency band of the residing domain. Furthermore, the probe response frame may have an information field that indicates the maximum transmission power allowed for the channel.
Assume, on the other hand, that a probe response frame is not received for the scanned channel within a certain time limit from the transmission of a probe frame, which is very short compared to the time limit in passive channel scanning. Then, the channel is determined by the wireless station to be unavailable. But, as in the passive scan, there can be two different reasons. One reason is that the channel is outside of the legal frequency band of the residing domain. The other is that there are currently no access points using the channel even though the channel is within the legal frequency band. By transmitting probe frames on all channels that are used in the WLAN system, a wireless station can collect all the information on channels that are currently supported by access points and within the legal frequency band of the residing domain.
The difference between an active scan and a passive scan is that an active scan actively provokes access points to transmit the probe response, while a passive scan waits passively for the beacon to be transmitted by access points. As a result, an active scan shows quite different characteristics from those of a passive scan. Since an active scan involves actual transmission of a probe frame, it may violate the regulations of the residing domain. For example, the channel to which the probe frame was transmitted may not be within the legal frequency band, or the transmission power of the probe frame may exceed the regulated maximum transmission power. In this sense, active channel scanning is inappropriate for safe domain-aware channel scanning. The main great advantage of active channel scanning, however, is that it usually takes less time than passive channel scanning does. This is mainly because, while the beacon interval is relatively large, the probe frame and probe response frame can be sent in shorter time. Active channel scanning, therefore, is desirable fast channel scanning such as in a handover situation.
However, neither passive channel scanning nor active channel scanning can be used for both safe and fast domain-aware channel scanning. Although the passive channel scanning is advantageous in terms of safely observing network regulations, such as maximum transmission power, it is slow, making smooth handover difficult. Active channel scanning, although fast, cannot be used in safe domain-aware channel scanning, since it may violate the regulations of the residing domain.
To improve the situation, IEEE 802.11d proposes transmitting the country code and the regulatory information, as shown in FIG. 3, in the beacon and probe response frames of IEEE802.11 WLAN. The element identifier indicates that this information is related to the domain information. The length of the information element is variable, as the element may contain more than one triplet, for example comprising the First Channel Number, Number of Channels, and Maximum Transmit Power Level fields. The Country String field of the element shall describe the country name and is 3 octets in length. The First Channel Number field shall indicate the lowest channel number in the sub-band described in this information element. The Number of Channels field of the sub-element shall indicate the number of channels in the sub-band. The group of channels described by each pair of the First Channel Number and the Number of Channels fields preferably shall not overlap and shall be monotonically increasing in channel numbers. The Maximum Transmit Power Level field shall indicate the maximum power, in dBm, allowed to be transmitted for the sub-band. The Pad field is for padding and shall have 0 or 1 octet in length.
According to IEEE 802.11d, when a wireless station has lost a connection with an access point, it shall passively scan to learn at least one valid channel, i.e. a channel upon which it detects IEEE 802.11 frames. Once the wireless station has acquired the information so that it is able to meet the transmit requirements of the regulatory domain, it shall transmit a probe request to an access point to gain the additional regulatory domain information contained in the probe response frame, unless the information was previously received in a beacon frame. The wireless station then has sufficient information available for operation in the regulatory domain.
While guaranteeing safety by using passive scan whenever a wireless station has lost connection with its access point and also reducing scanning time by using active scan after the regulatory information becomes available, IEEE 802.11d has a number of disadvantages. For example, scanning time for wireless stations staying within a domain is still long. In order to avoid possible regulation violations due to domain changes, IEEE 802.11d uses passive scan until valid domain information is obtained. However, the scanning time of a passive scan in the beginning is still significant for a wireless station that moves around but stays within a domain. Although the scanning time for IEEE 802.11d is better on the average than in the all-passive scan case, the worst case performance of IEEE 802.11d may be similar to the all-passive scan case.
Suppose a wireless station is connected to an access point of a WLAN system having N channels via Channel 1. After the wireless station has lost a connection with the access point, it tries to find a new channel. Assuming that Channel N is the only channel available around the station and that the channel scanning happens to be in increasing order of channel number, then the scanning time would be as much as (N−1) times as long as the beacon interval, which is the same as in the case of all-passive scan. Considering the number of wireless stations staying within a regulatory domain, which would be much larger than the stations crossing the regulatory domain boundary, the scanning speed cannot be sacrificed for the domain-aware roaming capability.
Further, IEEE 802.11d does not consider domain-independent channels. Some channels of a WLAN may be common to several neighboring domains or even for all participating domains. Therefore, there is no danger of violating regulations in transmitting a probe request frame on this kind of domain-independent channel. Accordingly, the domain independent channels can be actively scanned without waiting for domain information. IEEE 802.11d, however, does not consider domain independent channels, and passively scans domain-independent channels.
Therefore, a safe and fast domain-aware channel scan method and system are needed. The channel scanning method must preferably provide a fast scanning speed for the wireless stations staying within a regulatory domain. The channel scanning method must also enable wireless stations crossing the domain boundary to observe the regulation of the new domain.
Accordingly, there is a need for an improved wireless station and a method and system of scanning channels in a wireless communication network.